The most efficient protection against influenza infection is vaccination against circulating strains and it is important to produce influenza viruses for vaccine production as quickly as possible.
Wild-type influenza viruses often grow to low titres in eggs and cell culture. In order to obtain a better-growing virus strain for vaccine production it is currently common practice to reassort the circulating vaccine strain with a faster-growing high-yield donor strain. This can be achieved by co-infecting a culture host with the circulating influenza strain (the vaccine strain) and the high-yield donor strain and selecting for reassortant viruses which contain the hemagglutinin (HA) and neuraminidase (NA) segments from the vaccine strain and the other viral segments (i.e. those encoding PB1, PB2, PA, NP, M1, M2, NS1 and NS2) from the donor strain. Another approach is to reassort the influenza viruses by reverse genetics (see, for example references 1 and 2).
Reference 3 reports that a reassortant influenza virus containing a PB1 gene segment from A/Texas/1/77, the HA and NA segments from A/New Caledonia/20/99, a modified PA segment derived from A/Puerto Rico/8/34 and the remaining viral segments from A/Puerto Rico/8/34 shows increased growth in cells.
There are currently only a limited number of donor strains for reassorting influenza viruses for vaccine manufacture, and the strain most commonly used is the A/Puerto Rico/8/34 (A/PR/8/34) strain. However, reassortant influenza viruses comprising A/PR/8/34 backbone segments do not always grow sufficiently well to ensure efficient vaccine manufacture. Thus, there is a need in the art to provide further and improved donor strains for influenza virus reassortment.